What happened
On July 11, 2009, at 07:34 UTC, an Airbus A319, registration OK-NEP, operated by CSA, experienced a significant loss of separation with an ATR42, registration OK-JFJ, near the SUNER waypoint.
The incident began when air traffic control (NSEL EC) instructed the A319 to proceed direct to the Final Approach Fix (FAF) for Runway 24 and descend to FL 160. While the crew focused on the change in arrival track, they failed to properly note the new altitude assignment. Based on previous experience, the commander assumed a descent to FL 150 was required and instructed the pilot flying to set FL 150 on the Flight Control Unit (FCU).
As the aircraft descended through FL 175 at a rate of 3,700 ft/min, the TCAS system issued a "TRAFFIC - TRAFFIC" alert, identifying the ATR42 in a 9 o'clock position. At 07:35:30, the vertical separation between the two aircraft had dropped to only 200ft, with a horizontal separation of 1.12NM, both of which were well below the required regulatory minima. The crew eventually corrected the altitude to FL 160 following an inquiry from air traffic control.
The investigation
The ÚZPLN investigation examined flight data recorders, cockpit voice recorders, radar data, and crew testimonies. The investigation focused on the crew's adherence to standard operating procedures (SOPs) regarding altitude changes and the communication between the pilot flying and the pilot non-flying. The investigation also reviewed the air traffic controller's instructions and the technical status of both aircraft.
Findings
- The crew of the A319 did not immediately set the assigned altitude on the FCU following the ATC instruction.
- The crew failed to follow established company standards and procedures for altitude changes, specifically the requirement for the pilot flying to announce the change ("FLIGHT LEVEL... SET") and the pilot non-flying to verify it ("CHECK").
- The pilot flying expressed doubts regarding the assigned altitude, but the pilot non-flying demonstrated insufficient assertiveness in challenging the commander's incorrect instruction.
- The vertical separation dropped to 200ft, representing less than 50% of the required 1,000ft minimum.
- The technical condition of both aircraft and the meteorological situation were not contributing factors to the incident.